In an electrically powered space conditioning system with heating and cooling modes it is desirable to be able to use electrical energy at off-peak hours for two principal reasons. One is to reduce the peak demand on the utility to thereby limit the need for construction of new generating capacity. Using energy at off-peak hours increases the likelihood that the energy will be available and reduces the chance of a power outage. Also, the consumer does not need to pay the cost of amortizing the additional generating capacity. The second reason is that more favorable rates for the power can be obtained by the consumer from many utilities for such off-peak usage, and such rate structures are likely to be more widely obtainable in the future.
In order to achieve this goal, one must include some form of storage capacity in the system. A system which is designed to cool a space during the day and to use energy during the night can satisfy the off-peak energy usage requirements, but must include a cool storage facility of some kind in order to be able to cool the space during peak heat, and the demands on the storage are rather severe. While it is relatively easy to store heat, it is not so easy to store cold.
Cold storage can be described in terms of ton-hours which is a measure of the equivalent cooling capacity of a refrigeration system. One ton-hour equals 12,000 BTU. It is not unusual for a moderate home of recent construction to require 28 ton-hours of cooling during a hot summer day in a southern climate.
While it might be possible to supply a very large pool of water and to chill that water during the night so that it can be used to cool the house during the day, the size of the storage facility which would be necessary to provide significant assistance with the cooling load during the day is simply not practical for most residences and small commercial structures. Additionally, such a cool pool would only deal with lowering the temperature within the space and may not be effective to dehumidify which is at least as important as cooling. In order for water or some similar substance to be usable to dehumidify over a reasonable range of conditions, its temperature must be quite low, preferably below 48.degree.-50.degree. F. As a practical matter, it is not realistic to try to cool a large quantity of water (i.e., several hundred gallons) sufficiently so that it can cool and dehumidify a space such as a home for a significant portion of a hot day.
The obvious solution to this problem seems to be to chill the water until it becomes ice. By causing the water to go through a phase change to its solid state, much more energy per cubic foot is storable, thereby reducing the size of the storage facility required and also lowering the temperature of the liquid extractable from the storage so that it can effectively dehumidify. Other possibilities include solidification of eutectic salts and clathrate technologies. At the present time, both of these technologies present severe problems. Accordingly, they will not be discussed further. There are existing techniques for large commercial building cooling using ice by forming ice lumps (cubes, cylinders or the like) harvesting them into containers and using the resulting melt to cool the space. However, such systems require substantial maintenance and close supervision and are simply not usable, as a practical matter, for small buildings, particularly residences.
Although numerous efforts have been made to produce a satisfactory cold storage system, particularly using ice, no such system has proven to be practical or effective for a small commercial or residential structure. The primary problems with previous systems seem to lie in the inability to achieve reproducible results in creating and thawing the ice. Typically, such systems have involved flowing a chilling liquid, such as a refrigerant or the like, through tubes which are arranged in a water chamber. The goal has been to create ice surrounding the tubes and then, when it is necessary to extract heat from the space, to flow the fluid warmed by the space through the tubes, chilling the fluid with the ice previously formed. While this may seem simple, the fact that the ice acts as a partial insulator renders this technique essentially useless. In order to achieve reproducible results, it is necessary to totally thaw all of the ice in the system before commencing a re-freezing process and to freeze entirely before commencing thawing. This is a significant disadvantage because it requires a very inefficient use of the energy which drives the system.
Furthermore, it is important for the same storage facility to be usable for both heating and cooling, particularly in the residential context.